PEM fuel cell with charging chamber

ABSTRACT

Proposed is a PEM fuel cell that comprises a separator plate assembly with a charging chamber, which is partitioned off by a partition wall and via charging spots has a fluid connection to an adjoining cathode chamber. The partition wall is designed so that the depths of the charging channels in the charging chamber and the depths of the distribution channels in the adjoining cathode chambers change in such a way that the quantity of oxidant that is charged at a charging spot from the charging chamber into the cathode chamber can be fixed in advance. As a result, the charging of oxidant, which has not been humidified or only slightly, into the cathode chamber can be improved with regard to the curve of the relative humidity along the cathode. Also proposed is a method for manufacturing a separator plate assembly suitable for a PEM fuel cell.

The invention relates to a PEM fuel cell with charging chamber inaccordance with the preamble of claim 1, and to a method formanufacturing a separator plate assembly to be used in such a PEM fuelcell in accordance with the preamble of claim 12. The subject mattersaccording to the invention may find industrial application in the fieldof generating electric current for traction and/or the on-board powersupply system of vehicles.

The fundamental design of a polymer electrolyte membrane fuel cell (inshort: PEMFC; FC for Fuel Cell) adheres to the following principle: ThePEMFC comprises one membrane-electrode assembly (MEA) that is composedof one anode, one cathode, and a polymer electrolyte membrane (PEM)disposed in between. In turn, the MEA is arranged between two separatorplate units, whereby a separator plate unit arranged over the anode isequipped with fuel-distribution channels while a separator plate unitarranged over the cathode contains channels for the distribution ofoxidant, and whereby these channels face the MEA. Channels of this typewill be referred to as “distribution channels” hereinafter. Thedistribution channels form the anode chamber and cathode chamber,respectively. The electrodes, anode and cathode, are usually realized asgas diffusion electrodes (GDE). These electrodes serve the function ofconducting away the electric current generated in the electrochemicalreaction (e.g. 2H₂+O₂->2H₂O) (hereinafter: fuel cell reaction) and toallow the reactants, educts and products, to diffuse through. A GDEusually comprises at least one gas distribution layer, at least one gasdiffusion layer (GDL), and one catalyst layer, which faces the PEM andis where the electrochemical reaction takes place.

A fuel cell of this type is able to generate a high output of electriccurrent at comparatively low operating temperatures. Real fuel cells areusually stacked to form so-called fuel cell stacks (hereinafter: stacks)in order to achieve a high power output, whereby instead of monopolarseparator plates one employs bipolar separator plates (bipolar plates)so that monopolar separator plates only serve as stack end plates. Theseparator plates may consist of two or more partial plates that form anassembly and are therefore referred to as “separator plate assemblies”.The term “separator plate assembly” hereinafter is to denote all of theabove-mentioned plates and plate assemblies. Accordingly, a separatorplate assembly may comprise a single plate or may be assembled from oneor more partial plates.

As reactants one employs one fuel and one oxidant. In most cases oneuses gaseous reactants (reaction gases), e.g. H₂ or a H₂-containing gas(e.g. reformate gas) as fuel and O₂ or an O₂-containing gas (e.g. air)as oxidant. The term reactants is to be understood as referring to allof the substances participating in the electrochemical reaction, i.e.also the reaction products, such as H₂O.

Some current PEMs require a certain water content in order to achieveadequate ion conductivity. This is particularly true of PEMs thatconsist of materials based on fluorinated sulphuric acids, such asNafion. For this reason, PEMs of this type are usually humidified byhumidifying the reactants before these are supplied to a PEMFC. Thedisadvantage of humidification is the associated complexity and theadditionally necessary equipment (e.g. humidifier), which represents anobstacle in the striving for a very simple operating process and a verycompact design for a fuel cell system.

A further disadvantage of humidification is that the level ofhumidification is difficult to adjust (humidification problem). Forexample, if one humidifies not enough or not at all and trusts that thewater produced in the fuel cell reaction (product water) is sufficientto adequately humidify the PEM, then one faces the problem—especially onthe cathode side—that in the vicinity of the intake port for oxidant(preferably air) there exists a tendency for water to evaporate while atthe same time the oxidant at this location is comparatively dry, so thatthe PEM in this region is particularly prone to dry out. Drying out notonly results in the loss of ion conductivity, but can also causemechanical damage to the PEM, e.g. due to cracks.

While flowing through the channels of the separator plate assembly, theoxidant then absorbs product water from the MEA, which increases itsrelative humidity and progressively reduces the humidification problem,until this problem finally vanishes or under unfavourable conditionseven reverses into the opposite problem, a water removal problem.

The gist of the water removal problem is that the oxidant—on its wayfrom the intake port to the exit port—grows progressively more humid dueto absorbing product water and consequently can absorb and thus removeless and less product water, so that at the exit port or in its vicinityone may even face the situation that product water can not be adequatelyremoved. Now the product water condenses under unfavourable conditionsand can block important paths for the supply and removal of reactants toand from the reaction centres, which adversely affects the fuel cellreaction and reduces the output of the fuel cell.

The water removal problem occurs especially if the oxidant is humidifiedbefore entering a fuel cell. As a result of the humidification, the MEAin the region of the intake port is moist enough to prevent drying outof the PEM, however in this case the oxidant right from the start canabsorb less product water, so that the moisture level of the oxidantwill quickly be too high to adequately absorb and remove product water.This can easily impair the fuel cell reaction.

For this reason, in the case of conventional fuel cells withhumidification-dependent PEMs, one usually has to find a compromisebetween adequate humidification at the cathode inlet and adequate waterremoval at the cathode outlet.

As a solution to the humidification problem, the German Patentapplication DE 103 46 594 A1 of this applicant describes a fuel cellthat comprises two fluid chambers (one cathode chamber and one anodechamber), whereby a further fluid chamber is partitioned off in one ofthe fluid chambers. For simplicity and in view of the humidificationproblem to be dealt with, we will in the following only deal with thecase where the additional chamber is partitioned off the cathodechamber. But it is naturally also possible that one or even severalchambers are partitioned off both the cathode chamber and the anodechamber, if a technical reason exists to do so. In the following, thisseparated additional chamber will be referred to as “charging chamber”in accordance with its intended function. The dividing wall may be ametal sheet, which is realized similar to a partial separator plate andis arranged on the cathode side of the separator plate assembly. Themetal sheet will possess permeable spots, e.g. bore holes, which createa fluid connection from the charging chamber to the cathode chamber, sothat a fluid (the oxidant) flowing in the charging chamber can reach thecathode chamber through the permeable spots and in this manner can beinjected or charged (hereinafter: charging) into the cathode chamber. Inthe following, these spots will be referred to by the fitting term“charging spots”. The charging chamber may extend over only a portion ofthe cathode chamber, preferably only over one third. With the help ofthe charging chamber it becomes possible to spatially distribute andthus break up the supply of oxidant to the cathode chamber over a largerregion. As a result, not the entire volume of oxidant required for thefuel cell reaction will flow over the especially drying-sensitive partof the PEM in the region of the cathode inlet (intake region), but in abest case only a volume that can be electrochemically converted in thisregion. Since only a small amount of oxidant is charged in at thebeginning, only little water is used for humidification of the PEM, inthe optimum case less than is generated in this region as a result ofthe fuel cell reaction. Further downstream, before a further chargingspot, the oxidant already will be partially humidified as a result ofthe water created in the fuel cell reaction, so that the charging of dryoxidant is even less detrimental than at the preceding injection. Thistrend continues for the following charging spots, so that in sum totalthe fuel cell can use oxidant that has been humidified only to a smalldegree or not at all, without any detrimental effect from drying-out onthe PEM or its function. Thus, in a best-case scenario it becomespossible to eliminate the humidification and water recovery systems thatup to now are needed for PEM fuel cell systems, which represents asignificant simplification of such PEM fuel cell systems.

Even though DE 103 4 6 594 A1 with this approach provides the principleof a solution to the humidification problem, there exists further needfor improvement, e.g. to effect an exact charging of the proper amountof oxidant at the proper locations, without the need of having to employa complex control unit for this purpose.

Thus, one problem definition of the present invention is to furtherdevelop a PEM fuel cell of the above-mentioned type so that the chargingof oxidant, which has been humidified not at all or only to a smalldegree, into the cathode chamber can be further improved in a simplemanner with regard to the relative humidity curve along the cathode.

A further problem definition of the present invention is to specify amethod for manufacturing a component part for a PEM fuel cell, wherebywith the help of this part the charging of oxidant, which has beenhumidified not at all or only to a small degree, into the cathodechamber can be improved in a simple manner with regard to the curve ofthe relative humidity along the cathode.

These problems are solved by the subject matters defined in the claims.Preferred embodiments are defined in the dependent claims.

Accordingly, a first subject matter of the present invention is a PEMfuel cell, with one membrane-electrode assembly (MEA), one cathodechamber, which borders on the cathode of the MEA and on its side facingthe cathode is partly bordered by a separator plate assembly and partlyby a partition wall, which is arranged on the cathode side of theseparator plate assembly and joined to the assembly by for examplewelding, whereby the partition wall separates at least one additionalchamber (charging chamber) from the cathode chamber (i.e. the chargingchamber is bordered by the partition wall on the cathode side and by theseparator plate assembly on the opposite side), and whereby the at leastone charging chamber possesses in at least one position (charging spot)a fluid connection to the adjoining cathode chamber, so that oxidant canbe charged from the charging chamber through the charging spots into theadjoining cathode chamber.

The partition wall of the invention is designed so that the depths ofthe charging channels in the charging chamber and the distributionchannels in the adjoining cathode chamber change in such a manner thatthe quantity of oxidant that is charged at a charging spot from thecharging chamber into the cathode chamber can be fixed in advance.

If the partition wall does not cover the entire channel system on thecathode side, then the partition wall forms part of the distributionchannels, whereas the separator plate assembly forms the chargingchannels in the region of the partition wall and forms the remainingpart of the distribution channels outside of the region of the partitionwall.

If the partition wall covers the entire channel system on the cathodeside, then the partition wall forms the distribution channels, while theseparator plate assembly forms the charging channels.

The term “fixed in advance” in the context of the present invention isto be understood as to mean that the depth of the channels of theseparator plate assemblies are dimensioned or specified in advance insuch a manner so that when flows pass through the cathode chamber andcharging chamber during operation of the fuel cell, specific pressureswill prevail at specific charging spots, which cause specific quantitiesof gas to flow from the charging chamber through the charging spots intothe cathode chamber.

Due to the varying depths of the distribution channels and the chargingchannels, the fuel cell according to the invention is designed so thatcharging of oxidant—not humidified or humidified only slightly—from thecharging chamber into the cathode chamber takes place selectively sothat an improved curve of the relative humidity along the cathode isachieved in a simple manner. The improved curve is illustrated by thefact that along the region in which the charging takes place (chargingregion) an adequate humidification of the PEM is given at all positionsand moreover that no liquid water is formed there, which could adverselyaffect the fuel cell reaction.

It is practical for the depths of the distribution channels to becomplementary to the depths of the charging channels. This means that atpositions where the oxidant distribution channels possess a narrowdepth, the adjoining charging channels will possess a correspondinglygreater depth, and vice versa. That is how the pressures of the fluids(oxidant) flowing in the two chambers can be selectively influenced sothat pressure equalization will take place through the charging spotsand predetermined quantities of oxidant will selectively and in a dosedmanner flow into the cathode chamber.

The depth of the distribution channels preferably increases along theflow direction from the charging region towards the cathode outlet, andcomplementarily the depth of the charging channels decreases along theflow direction from the oxidant port to the end of the charging region.I.e. in this embodiment the charging channels become shallower along theflow direction of the oxidant, whereas the distribution channels startshallow, become deeper along the flow direction, and outside of thecharging region continue at a constant depth to the cathode outlet, forexample. It should be noted in this connection that the charging regionmight extend over the entire length of the channels formed in theseparator plate assembly, so that the end of the charging region will belocated in the region of the cathode outlet. This can create pressureconditions in the charging chamber and the adjoining cathode chamberthat ensure a flow behaviour of the oxidant from the charging chamberinto the cathode chamber that is advantageous with regard to the curveof the relative humidity in this region (charging region).

In a further development of the fuel cell according to the invention,the partition wall covers at least part of the length of the channelsformed on the cathode side of the separator plate assembly and thusisolates them from the cathode chamber, thus forming the chargingchamber in the region of the cover (cover region). In this connection itshould be noted that the channels that are formed on the separator plateassembly and that are positioned in the cover region are chargingchannels, whereas any continuation of channels outside of the coverregion will be part of the distribution channels.

It is preferable that the partition wall be arranged in the region ofthe oxidant port of the separator plate assembly. This simplifies thecreation of a fluid connection between the oxidant port and the chargingchamber and also simplifies the sealing of this connection against thesurroundings. Furthermore, a channel structure existing in the portregion can be used as oxidant feed into the charging region.

Further it is preferred that the partition wall extend from the oxidantport to at most the end of the first third of the length of the channelsformed on the cathode side of the separator plate assembly, even morepreferred to at most the end of the second third of the channels formedon the cathode side of the separator plate assembly, and especiallypreferred to at most the end of the third third of the channels formedon the cathode side of the separator plate assembly. Even though themechanical complexity increases as the extent of the partition wallincreases, the complexity required for the humidification of the oxidantdecreases as the extent of the partition wall increases. For example,the required dimensions of a humidification device will be smaller or iteven becomes possible to eliminate the humidification device, inparticular if the partition wall extends over the entire length of thechannels formed on the cathode side of the separator plate assembly.

In a further development of the fuel cell according to the invention, inthe region of the cover, the channels formed on the cathode side of theseparator plate assembly are separated by the partition wall from theoxidant port in such a manner, that a fluid connection to the oxidantport essentially only exists via the charging spots and the chargingchamber. Thus, the charging chamber completely replaces the cathodeintake region of conventional fuel cells. This allows a more precisecontrol or setting of the charging of oxidant into the cathode chamber.

In another further development of the fuel cell according to theinvention, the height of the ridges between the channels of thepartition wall and the height of the ridges between the channels of theadjoining separator plate assembly are complementary to each other, sothat the overall height of the separator plate assembly in the coverregion corresponds to the overall height of the separator plate assemblyoutside of the cover region. This simplifies the stacking and clampingof the invention's fuel cell into a stack and ensures that the clampingforces are introduced into the stack in a way that does not deform thefluid chambers, which could have a negative impact on reactant flow,charging, and the curve of the relative humidity.

In this, it is preferred for the height of the ridges of the channelsformed on the cathode side of the separator plate assembly to be lowerin the region covered by the partition wall by the thickness of thematerial of the partition wall. Thus, the separator plate assemblyassembled from the partition wall and the partial platespossesses—ignoring the depth of its channels for now—a uniform thicknessalong its surface. This also contributes to good stacking and clampingcharacteristics of the fuel cell according to the invention.

In one embodiment of the invention's fuel cell the partition wall ismade up of a metal plate, preferably a metal sheet, into whichpreferably a channel structure is stamped, which forms the distributionchannels in the region of the partition wall. Metal plates, inparticular metal sheets, offer the advantage of excellent and precisemachinability, particularly by embossing, so that the channels, inparticular the depth of the channels and the height of the ridges inbetween, can be incorporated very precisely.

In this, it is preferred if the embossed metal sheet possesses boreholes that are intended as charging spots, whereby the bore holespreferably are arranged at the bottom of the distribution channels. Itis practical for the diameters of the bore holes to be dimensioned sothat the pressure loss of flowing through the bore holes issignificantly higher than the pressure loss of flowing along thecharging channel. In the present case, bore holes with a diameter of 0.3mm, for example, were found to be suitable. The diameters of all boreholes should be essentially equal. As a result, the gas volume flowingthrough a bore hole per time unit is approximately equal for all boreholes, and the charged oxidant volume can be set in a simple manner viathe number of bore holes. Bore holes of this type can be realized verysimply and precisely, which contributes to the improved precision of thecharging.

A further subject matter of the present invention is a method formanufacturing a separator plate assembly to be used in a fuel cell asdescribed above, whereby the separator plate assembly comprises onepartial plate on the anode side, one partial plate on the cathode side,and one partition wall arranged on the cathode-side partial plate.According to this invention, one at first joins the partition wall withthe cathode-side partial plate, whereupon one attaches the anodesidepartial plate to the cathode-side partial plate.

This prevents the occurrence of joining problems, which can be caused bythe third partial plate of the separator plate assembly, the partitionwall. If one at first joins the anode-side and the cathode-side partialplates and only afterwards connects the partition wall to thecathode-side partial plate, one faces the risk that the anode-sidepartial plate and/or the cooling channels located between the anode-sideand cathode-side partial plates will be damaged during the joiningprocess because joining connections for the partition wall must bepositioned in the region of the already joined partial plates. This riskdoes not exist in the method according to the invention, since thejoining connections for the anode-side partial plate can be put outsideof the region of the already joined partition wall and cathode-sidepartial plate.

The invention's method can be used to manufacture a component part,namely a separator plate assembly for a PEM fuel cell, which can help toimprove in a simple manner the charging of oxidant, which has not beenhumidified or only slightly, into the cathode chamber with regard to thecurve of the relative humidity along the cathode.

In this, it is preferable to join the partial plates and the partitionwall by welding, since welding allows in an uncomplicated manner thecreation of even joining connections with very complex geometry (complexweld paths), in particular for metal plates.

In one variant of the invention's method, the welding spots are placedthrough charging spots. Additional welding points outside of the weldseams joining the anode-side and cathode-side partial plates can improvethe electrical connection between the two partial plates. However, inthe case at hand part of the cathode-side partial plate is covered bythe partition wall and thus no longer accessible for placing a weldpoint to improve the electrical contact between the anode-side andcathode-side partial plate. An exception to this are the charging spotsin the partition wall, e.g. the bore holes. The cathode-side partialplate that is covered by the partition wall is still accessible throughthese charging spots, so that for example a laser can be used to place aweld point there to improve the electrical contact between the partialplates.

The invention will be explained in more detail in the following. Forthis purpose, concrete embodiment examples of the invention areillustrated in a simplified manner in the figures and are explained inmore detail in the following description.

FIG. 1 shows a schematic exploded view of a state of the art fuel cell;

FIG. 2 shows a detail of FIG. 1;

FIG. 3 shows a schematic representation of a conventional separatorplate assembly: a) top view onto the cathode-side partial plate; b) topview onto the anode-side partial plate;

FIG. 4 shows the curve of the relative humidity in a channel system of astate of the art fuel cell with a partial plate according to FIG. 3 a);

FIG. 5 shows a separator plate assembly with partition wall for a fuelcell according to the invention;

FIG. 6 shows two sections through a separator plate assembly withpartition wall: a) in the region of the cathode inlet; b) in the regionof the end of the charging chamber;

FIG. 7 shows the curve of the relative humidity along a cathode channelfor a first embodiment of the invention;

FIG. 8 shows the curve of the relative humidity along a cathode channelfor a second embodiment of the invention.

Only the features that are essential for understanding the invention arelabelled in detail in the figures.

FIG. 1 shows an exploded view of the components of a conventional PEMfuel cell (1) that are essential within the framework of the presentinvention: The fuel cell (1) comprises one PEM (2), that is disposedbetween one anode (3) and one cathode (4) and joined together with theseforms a MEA. The present fuel cell (1) is part of a stack (notillustrated), so that on the anode side and on the cathode side the MEAis bordered by bipolar separator plate assemblies (bipolar plates), foreach of which only one of two partial plates (5, 6) is shown, namely theanode-side partial plate (5) and the cathode-side partial plate (6),respectively. If the fuel cell (1) is an individual cell that is notpart of a stack, then (5) and (6) will be end plates. If the fuel cell(1) is an individual cell at the end of a stack, then (5) or (6) will bean end plate. The partial plates' (5, 6) surfaces that face an electrodepossess a structured pattern, namely a channel system (7, 8), whereby inthe assembled fuel cell the anode-side channel system (7) forms theanode chamber (9) and the cathode-side channel system (8) forms thecathode chamber (10). The electrode chambers (9, 10) are sealedfluid-tight against undesired leaks by sealing rings (11). The partialplates (5, 6) are joined in a stack to form a separator plate assembly.In the present example, the partial plates (5, 6) possess a structuredpattern/channel system (7, 8) only on the respective surface that facesthe electrode. But they can also be equipped with a channel system onthe opposite surfaces, so that after assembly a further channel systemis formed between them, which can be used to conduct a coolant forcooling purposes. The label (12) identifies a detail that is shown on alarger scale in FIG. 2.

Such a fuel cell (1) comprises further components, but these are knownto a person skilled in the art and are not illustrated here for brevity.For example, these include outlet and intake ports for the reactants andthe coolant, if applicable, further sealing elements, inlays for thefeeding of reactants from the ports to the channel systems, and soforth.

FIG. 2 shows the detail (12) of FIG. 1 on a larger scale. Shown is theintake region of a distribution channel (15) of the channel system (8)of the separator partial plate (6), as well as the depth (13) of thedistribution channel (15), whereby the depth (13) in this conventionalfuel cell is constant along the cathode-side surface of the separatorpartial plate (6). The label (14) denotes a ridge of the channel system(8).

FIG. 3 a) shows a cathode-side partial plate (6) of a separator plateassembly of a conventional fuel cell while FIG. 3 b) shows an anode-sidepartial plate (5). We will only deal with the cathode-side partial plate(6) in the following. It comprises (among other features) one intakeport (16) for an oxidant, one meandering channel system (8), and oneoutlet port (17) for the oxidant. The depth of the distribution channels(15) of the channel system (8) is constant along the entire length ofthe distribution channels (15).

FIG. 4 schematically illustrates the curve of the relative humidity f(18) in a channel system (8) of a conventional fuel cell (1) asdescribed above in FIGS. 1 to 3, whereby L represents the length of adistribution channel (15, compare FIG. 2). As oxidant in the presentcase one uses partially humidified air, which enters into thedistribution channel at L=0 with a relative humidity f_(E) of 53% (atapproximately 75° C. and 1 bar gauge) (hereinafter: intake humidity).The relative humidity f of the air subsequently at first rises along thechannel as a result of the fuel cell reaction, until the saturationpoint (19) is reached at f=1 and L˜⅓. Up to the saturation point (19),the air is a single-phase system, whereas it becomes a two-phase system(gaseous air, liquid water) after the saturation point (19). Problemscan arise in the cathode intake region between L=0 and L˜⅓, since herethe PEM has the tendency of drying out because of the low relativehumidity f of the air. The objective of the present invention is toimprove the curve of the relative humidity f in this region to such adegree so that the drying risk will be reduced or even eliminated.

FIG. 5 shows part of a separator plate assembly of a fuel cell accordingto the invention, with an oxidant port (20), a fuel port (21), a coolantport (22), and a cathode-side channel system (8). Shown is a perspectivetop view onto a cathode-side partial plate (6) of the separator plateassembly. The separator plate assembly comprises a partition wall (23)that is realized as metal sheet and is welded onto the separator plateassembly. The partition wall (23) separates a charging chamber (compareFIG. 6) from the cathode chamber, i.e. it is arranged between thecathode and the separator plate assembly that faces the cathode. Thepartition wall (23) possesses a channel system that is formed so thatthe depths of the charging channels in the charging chamber and of thedistribution channels in the adjoining cathode chambers change in such away that the quantity of oxidant that is charged at a charging spot (24)from the charging chamber into the cathode chamber can be fixed inadvance (compare FIG. 6). In the present example, the partition wall(23) is a metal sheet, into which a channel structure has been stamped,which forms the distribution channels in the region of the partitionwall (23). The partition wall (23) is arranged in the region of theoxidant port (20) and covers approximately one third of the length ofthe cathode-side channel system (8). But it is preferable for thepartition wall (23) to cover the entire length of the cathode-sidechannel system (8), i.e. the separator plate assembly's entire regionthat adjoins to the electrochemically active area of the adjoining MBA(not illustrated). The partition wall (23) is further equipped with boreholes that are arranged in the bottom of the distribution channels (15)and that are intended as charging spots (24) for air from the chargingchamber into the cathode chamber. These bore holes possess a diameter of0.3 mm. In the region where they are covered by the partition wall (23)(covered region), the channels (15) formed on the cathode-side of theseparator plate assembly are separated from the oxidant port (20) insuch a way that a fluid connection to the oxidant port (20) existsessentially only via the charging sports (24) and the charging chamber.

FIG. 6 shows a schematic representation of two sections through theseparator plate assembly with partition wall (23) for a fuel cellaccording to the invention. FIG. 6 a) shows a section through thecathode intake region while FIG. 6 b) shows a section through the endregion of the charging chamber. If the partition wall (23) covers theentire length of the channels formed on the cathode side of theseparator plate assembly, then the end of the charging chamber will belocated in the region of the cathode outlet. It is also evident that theseparator plate assembly comprises two partial plates in addition to thepartition wall (23), in particular an anode-side partial plate (5) withan anode-side channel system (7) and a cathode-side partial plate (6)with a cathode-side channel system (8), whereby a cooling channel system(25) is formed between the two partial plates. In the cathode chamber,the partition wall forms distribution channels (15), while in thecharging chamber (26) it delimits the charging channels (27) on thecathode side. The depths of the distribution channels (15) of thepartition wall (23) are labelled (13) for the region of the sectionthrough the cathode intake region and (13′) for the region of thesection through the end region of the charging chamber. The depths (13,13′) increase from the cathode intake region towards the cathode outletregion/end region of the charging chamber, i.e. in the flow direction ofthe oxidant. Since the partition wall (23) is arranged on the separatorplate assembly, as illustrated in FIG. 6, the depths (13″, 13′″) of thecharging channels (27) decrease along the same direction and to the samedegree, i.e. the depths (13, 13′) of the distribution channels (15) aredimensioned complementary with respect to the depths (13′, 13′″) of thecharging channels (27). A suitable dimensioning of the channel depths(13 to 13′″) allows the creation of pressure conditions in the chargingchamber and the adjoining cathode chamber that cause a pressureequalization to occur through the charging spots (not illustrated),which causes oxidant to be charged selectively from the charging chamberinto the adjacent cathode chamber. In this way, the charging can beselectively set in advance with the help of the specially designedstructure depth of the partition wall.

FIG. 7 shows a curve of the relative humidity f (18, 18′, 18″) along acathode channel for a first embodiment of the invention's fuel cell. Theshown chart is structured analogously to the chart shown in FIG. 4,except that the channel length L here is given in millimeters instead offractions of the total channel length. In the illustrated case, thecharging region is designed so that saturation (f=1) is reached but notexceeded at the charging spots (24), of which only one is labelled (24)for a clearer presentation. Shown as a comparison below the saturationpoint (19) are two branches (18′, 18″) of the curve of the relativehumidity f in this region: The upper branch (18′) possesses asaw-tooth-like curve and at the saturation point (19) joins the curve(18), which is already known from FIG. 4. The curves (18′) and (18)together form the curve of the relative humidity f (18, 18′) in a fuelcell according to the invention. As a comparison, the lower branch (18″)shows the curve of the relative humidity f in this region in aconventional fuel cell. The lower branch (18″) also joins the curve (18)(already known from FIG. 4) at the saturation point (19). The curves(18″) and (18) together form the curve of the relative humidity f (18,18″) in a conventional fuel cell.

The saw-tooth-like curve of the upper branch (18′) of the relativehumidity f (18, 18′) in the invention's fuel cell can be explained asfollows: The not-humidified oxidant (ambient air) enters thedistribution channel at L=0 with an intake humidity f_(E). However,according to the invention, not the entire volume of oxidant requiredfor the fuel cell reaction enters here, but only a portion. The relativehumidity f of the oxidant strongly increases along the flow directiondue to the water produced in the fuel cell reaction. This rise isparticularly strong in comparison to a conventional fuel cell becausethe quantity of charged oxidant is lower than in conventional fuel cellswhile the production of product water is approximately equal, so thatthe ratio of partial pressure of water vapour to partial pressure ofoxidant is greater in the invention's fuel cell than in conventionalfuel cells. The relative humidity f drops abruptly at the first (alongthe flow direction) charging spot (24) because the charged oxidant iscomparatively dry, which shifts the ratio of partial pressure of watervapour to partial pressure of oxidant back in favour of the oxidant.Subsequently the relative humidity f again strongly rises along the flowdirection until once again oxidant is mixed in, etc., until the end ofthe charging region is reached (approximately at the location where thesaturation point (19) is reached). As a result, the relative humidity fin the region where the oxidant enters into the cathode chamber of aninvention's fuel cell can be increased in comparison to a conventionalfuel cell in such a way that the risk of a drying out of the PEM in thisregion is reduced or even eliminated. On the other hand, theintake/charging region (28) of the invention's fuel cell is designed sothat the relative humidity f at no point exceeds saturation (f=1). Thisensures that no liquid water is created in the charging region (28),which under unfavourable conditions could block the charging spots (24).Thus, in total the relative humidity f in the region where the oxidantenters into the cathode chamber of an invention's fuel cell can beincreased in comparison to a conventional fuel cell, without any risk ofdroplet formation or blocking of charging spots (24).

FIG. 8 shows the curve of the relative humidity f (18, 18′, 18″) along acathode channel for a second embodiment of the invention's fuel cell.The charging region of this fuel cell is in principle designed analogousto the one in FIG. 7, however the charging region (28) possessesadditional five groups of charging spots (24) (again, for clarity's sakeonly one charging spot (24) is labelled in detail, representing all ofthe sixteen illustrated ones). One notices that the number of chargingspots (24) per group increases along the flow direction of the oxidant(from left to right). This allows to further increase (in comparison toa conventional fuel cell) the relative humidity f in the region wherethe oxidant enters into the cathode chamber of a fuel cell according tothe invention, whereby the risk of droplet formation and blocking of thecharging spots (24) is low.

The curves of the relative humidity f shown here have been improved bythe above-described charging concept. The improvement is achieved byraising the local relative humidity f at the MEA in the region of thepartition wall and thereby achieving an improved conductivity of theMEA. This has a positive effect on the homogenization of the fuel cellreaction and consequently of the current density distribution, and as aresult also on the lifetime of the MEA and its PEM, and thus the entirefuel cell.

The invention has been explained on the basis of the depths of thecharging channels and of the adjacent distribution channels. However, onprinciple the invention can be applied analogously to geometric featuresof the channels other than the depth, such as for example their flowcross sections.

LIST OF REFERENCE NUMBERS

-   1 PEM fuel cell-   2 PEM-   3 Anode-   4 Cathode-   5 Anode-side partial plate-   6 Cathode-side partial plate-   7 Anode-side channel system-   8 Cathode-side channel system-   9 Anode chamber-   10 Cathode chamber-   11, 11′ Sealing ring-   13 Detail-   13, 13′ Channel depth-   14 Ridge-   15 Distribution channel-   16 Intake port for oxidant-   17 Outlet port for oxidant-   18 Curve of the relative humidity f (State of the art)-   18′ upper branch of the relative humidity curve below the saturation    point (Invention)-   18″ lower branch of the relative humidity curve below the saturation    point (State of the art)-   19 Saturation point-   20 Port for oxidant-   21 Port for fuel-   22 Port for coolant-   23 Partition wall-   24 Bore hole, charging spot-   25 Cooling channel system-   26 Charging chamber-   27 Charging channel-   28 Charging region-   f relative humidity-   f_(E) Intake humidity-   L Channel length

The invention claimed is:
 1. A PEM fuel cell, with a membrane-electrodeassembly (MEA), a cathode chamber, which borders on the cathode of theMEA and on its side facing the cathode is defined for one part by aseparator plate assembly and for the other part by a partition wall,which partition wall is arranged on the cathode side of the separatorplate assembly and is joined to the separator plate assembly, wherebythe partition wall separates at least one charging chamber from thecathode chamber and whereby the at least one charging chamber possessesin at least one charging spot a fluid connection to the adjoiningcathode chamber, so that oxidant can be charged from the chargingchamber through the charging spots into the adjoining cathode chamber,and wherein the depth of the distribution channels increases along theflow direction from the charging region to the cathode outlet, and,complementary to this, the depth of the charging channels decreasesalong the flow direction from the oxidant port to the end of thecharging region, such that the quantity of oxidant that is charged at acharging spot from the charging chamber into the cathode chamber can beset in advance.
 2. The PEM fuel cell of claim 1, wherein in the chargingregion charging spots are arranged in groups, and wherein the number ofcharging spots per group increases along the flow direction of theoxidant.
 3. The PEM fuel cell of claim 1, wherein the partition wallcovers and separates from the cathode chamber at least part of thelength of the channels formed on the cathode side of the separator plateassembly, which is how the charging chamber is created in the region ofthe cover.
 4. The PEM fuel cell of claim 3, wherein the channels formedon the cathode side of the separator plate assembly are in the coverregion separated from the oxidant port in such a way that a fluidconnection to the oxidant port exists essentially only via the chargingspots and the charging chamber.
 5. The PEM fuel cell of claim 3, whereinthe heights of the ridges between the channels of the partition wall andthe heights of the ridges between the channels of the adjoiningseparator plate assembly are formed complementary to each other in thecover region, so that the overall height of the separator plate assemblyin the cover region is equal to the overall height of the separatorplate assembly outside of the cover region.
 6. The PEM fuel cell ofclaim 3, wherein the height of the ridges of the channels formed on thecathode side of the separator plate assembly is reduced in the region ofthe cover by the value of the thickness of the material of the partitionwall.
 7. The PEM fuel cell of claim 1, wherein the partition wall isarranged in the region of the oxidant port of the separator plateassembly.
 8. The PEM fuel cell of claim 1, wherein the partition wallextends from the oxidant port to the end of the channels formed on thecathode side of the separator plate assembly.
 9. The PEM fuel cell ofclaim 1, wherein the partition wall is made of a metal sheet, into whicha channel structure is stamped, which forms the distribution channels inthe region of the partition wall.
 10. The PEM fuel cell of claim 9,wherein the stamped metal sheet possesses bore holes that are intendedas charging spots, whereby the bore holes are arranged in the bottom ofthe distribution channels.
 11. A method for manufacturing a separatorplate assembly for application in a fuel cell of claim 1, whereby theseparator plate assembly comprises one anode-side partial plate, onecathode-side partial plate, and one partition wall arranged on thecathode-side partial plate, wherein at first the partition wail isjoined to the cathode-side partial plate and subsequently the anode-sidepartial plate is joined to the cathode-side partial plate, wherein thepartial plates and the partition wall are joined by welding, and whereinwelding points are placed through charging spots.
 12. The PEM fuel cellof claim 1, wherein the partition wall covers and separates from thecathode chamber the entire length of the channels formed on the cathodeside of the separator plate assembly, which is how the charging chamberis created in the region of the cover.
 13. The PEM fuel cell of claim 1,wherein the partition wall extends from the oxidant port to at most theend of the first third of the length of the channels formed on thecathode side of the separator plate assembly.
 14. The PEM fuel cell ofclaim 1, wherein the partition wall extends from the oxidant port to atmost the end of the second third of the channels formed on the cathodeside of the separator plate assembly.